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Scientists operating the Fermi gamma wave telescope have published the results of a four-year study into the light that shines in the universe from current and past stars.

The experiment measures the light from the first stars formed after the Big Bang, the so-called extragalactic background light (EBL). This acts as a "fog" that interferes with gamma rays as they speed across the universe, and can split the rays into an electron and its antimatter counterpart, a positron, upon collision.

"The EBL is the ensemble of photons generated by all the stars and also all the black holes in the universe," said astrophysicist Marco Ajello of the SLAC National Accelerator Laboratory in California, who led the research.

"The EBL also includes the light of the first massive stars that ever shone. We have a fairly good knowledge of the light emitted by 'normal' stars. Thus, by measuring the EBL we are able to constrain the light of the first stars."

The team managed this by studying the effect of EBL on blazars, galactic nuclei in supermassive black holes that blast out gamma rays across the universe. They used over 150 blazars to draw up a map of the EBL around our Milky Way.

The findings also allowed the team to make a sensible guess at the mass of stars within the universe. They suggest that there are 1.4 stars per 100 billion cubic light-years, meaning the average distance between stars in the universe is about 4,150 light-years.

This might be one of the reasons no extraterrestrials have yet dropped round for a chat. As the late, great Douglas Adams put it:

"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." ®